At the moment the only Direct Storage mode that is supported is the write-out of data stored in the Transient Buffer Boards.

Transient Buffer Boards:

The Transient Buffer Boards (TBBs) contain the raw voltage signal of every dipole at the full time resolution and can be externally triggered to obtain this data. If triggered, the contents of the
boards are frozen and sent to the CEP/OLAP for further processing. The
minimum integration time is 5 ns for the 200 MHz clock (6.25 ns for the 160 MHz clock, currently unsupported), and the maximum bandwidth is 100 MHz.
The TBBs can dump at most 5 sec of data per event at full resolution.

TBB observations work currently with an external trigger provided by for example the
particle detector LORA. Plans for the future include continuous triggering
independently on pulses in LOFAR data itself. Such triggering improves with increased signal-to-noise ratio, thus a rather low noise level is needed.
Transient RFI conditions are such that -until the self-triggering
algorithm is improved to suppress these- most self-triggers are due to
local RFI sources.

Data Products

Data is stored as Raw Voltages per station in HDF5 format,
including some of the metadata. The software package to access the data and do
some processing (eg. FFT, RFI mitigation, ..) with python scripts,
PyCRTools, is available at CEP.

At the moment the automatic processing pipeline is not yet available on CEP for
TBB data. There is a
semi-automated Cosmic Ray pipeline in Nijmegen (~20s per event per
station). For processing VHECR events using this pipeline one should
contact the CRKSP group.

Performance Parameters

Currently for LORA triggered dumps, 2ms of data of all available core stations are written per TBB. The Table summarises the resources used for such a TBB dumb.

Apart from the overhead time, which is newarly a fixed number, the numbers scale linearly with dumptime.

Mode

Sampling Time

Data Rate

Data Volume

Data send rate to CEP

Overhead Time

Processing

(ns)

(MB/ms per station)

(GB/24 stations per

2ms)

(GB/60s per station)

(s)

timeseries

5

38.4

1.8

1

200

not available at CEP

The typical data size is 190 MB per station (for a core station) and two
polarizations for 5 ms of read-out. The data size depends on the number
of antennae used.

The maximum amount of data than can be dumped accounts for 5 seconds. For a 1.28 seconds
(with the current default time resolution of 5 ns for cosmic ray
observations). It takes 40 minutes to write a full second of TBB on
the storage nodes (with forthcoming improvements this may be reduced to 8
minutes).
The data send rate can be improved by a factor 6 by increasing the number of data paths (however, this will result in an higher data load which has not been tested well in piggy back mode).

A full dump of one second of data for one RCU is about 400 MB. A full
dump of one 48 antenna station therefore amounts to approximately
40 GB.

Characteristic Observation

Figure 1: Raw voltages across time.

For a typical TBB observation, Figure 1 shows the raw voltages of all antennas of one polarisation of
1 station, as recorded in the TBB, when a cosmic ray (7*10^17 eV) was
detected. The trigger to dump these data came from the LORA particle
detector.

The data is (phase) shifted in time to correct for delays between the signals of the different dipoles.